A variety of high bandwidth-efficiency technologies are being proposed in mobile communications systems in response to growing data communications demand. One of potentially high bandwidth-efficiency technologies is OFDMA (Orthogonal Frequency Division Multiple Access). OFDMA divides a communication area (cell area) into a plurality of cells and performs communications using the same frequencies across all the cells. The technology modulates data by OFDM and employs TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), or similar multiple access as its access scheme. OFDMA of course, in an isolated cell, shares a common wireless interface with the cell area, but is capable of achieving high-speed data communications.
A scheduling method for OFDMA systems is being proposed. In HSDPA (High Speed Downlink Packet Access) in W-CDMA (Wideband-Code Division Multiple Access) mobile communications system, communication terminals send the base station a CQI (Channel Quality Indicator; information on downlink condition for all subcarriers). The base station schedules packets on the basis of the CQIs reported for all subcarriers by the communication terminals. See non-patent document 1.
Patent document 1 discloses a method whereby the amount of uplink control information is reduced by organizing subcarriers into blocks and transmitting modulation information for each block (subcarrier number at a particular position in each block and modulation scheme designation information) from mobile stations to the base station. Furthermore, patent document 1 also ponders organizing subcarriers into blocks and according to changes in the environment of transmission paths, adaptively varying the number of subcarriers contained in a block. This method organizes adjacent subcarriers with a less-than-a-predetermined-threshold difference in reception power into blocks and controlling the number of blocks to a minimum, in order to lower the amount of uplink control information.
Non-patent document 2 proposes two methods. A first method organizes subcarriers into groups. All mobile stations calculate average reception quality for the subcarriers in each of the groups and feed back the average reception quality for each of the groups to the base station. A second method organizes subcarriers into groups. Each mobile station is assigned one group in advance and feeds back reception quality only for all the subcarriers in that group to the base station.
Patent Document 1: Japanese Unexamined Patent Publication (Tokukai) No. 2003-169036 (published Jun. 13, 2003)
Non-patent Document 1: Lucent Technologies, “Comments on frequency scheduling and joint power and rate optimization for OFDM,” 3GPP, TSG RAN WG1 Meeting #29, R1-02-1321, November 2002
Non-patent Document 2: NTT Docomo, “Physical Channels and Multiplexing in Evolved UTRA Downlink,” 3GPP, TSG RAN WG1 Ad Hoc on LTE R1-050590, June 2005
However, in scheduling data transmission in a system where each frequency channel (subcarrier) is allocated to a different mobile station as in the OFDMA system of non-patent document 1, reception quality for all mobile stations and over all frequency channels needs to be compared at a cycle at which the base station changes the allocation. Stated another way, all mobile stations have to periodically inform the base station of reception quality measurements for all frequency channels. That undesirably adds to the amount of uplink control information and overhead.
In addition, in the method of patent document 1, the size of blocks needs to be controlled according to the reception power of incoming subcarriers, and the mobile station needs to notify the base station of a subcarrier number at a particular position, when the subcarriers are organized into blocks. Also, simply organizing adjacent subcarriers with a less-than-a-predetermined-threshold difference in reception power into blocks as in patent document 1 could not sufficiently reduce the amount of information uploaded from the mobile stations to the base station. That might add to overhead.
Simple grouping as in non-patent document 2 could not sufficiently reduce the amount of information uploaded from the mobile stations to the base station. That might add to overhead.